Casing material for a solid electrolytic capacitor

ABSTRACT

A capacitor assembly that is capable of performing well under the conditions of high humidity and temperature (e.g., 85% relative humidity/85° C.) is provided. The capacitor assembly comprises a solid electrolytic capacitor element, an anode termination that is in electrical connection with the anode body, cathode termination that is in electrical connection with the solid electrolyte, and casing material that encapsulates the capacitor element and leaves exposed at least a portion of the anode termination and the cathode termination. The casing material is formed from an epoxy composition that contains one or more inorganic oxide fillers and a resinous material that includes one or more epoxy resins crosslinked with a co-reactant, wherein inorganic oxide fillers about 75 wt. % or more of the epoxy composition, and vitreous silica constitutes about 30 wt. % or more of the total weight of inorganic oxide fillers in the epoxy composition.

CROSS REFERENCE TO RELATED APPLICATION

The present application claims filing benefit of U.S. Provisional PatentApplication Ser. No. 62/422,077 having a filing date of Nov. 15, 2016,and which is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

Electrolytic capacitors (e.g., tantalum capacitors) are increasinglybeing used in the design of circuits due to their volumetric efficiency,reliability, and process compatibility. For example, one type ofcapacitor that has been developed is a solid electrolytic capacitor thatincludes a tantalum anode, dielectric layer, and conductive polymersolid electrolyte. To help protect the capacitor from the exteriorenvironment and provide it with good mechanical stability, it istypically encapsulated with an epoxy resin. While such epoxy resins aresuitable in most environments, they can nevertheless become problematicin certain circumstances. For example, certain types of conductivepolymer electrolytes (e.g., PEDT) are highly sensitive due to thetendency of such polymers to readily oxidize in the presence ofmoisture. Unfortunately, the presence of a very high level of moisture(e.g., 85% relative humidity) in the ambient environment can cause theepoxy resin to slowly weaken or degrade, which may eventually allow asmall amount of moisture to penetrate into the interior of thecapacitor. This is particularly problematic at high temperatures (e.g.,about 85° C.). Even in a small amount, the moisture can result inoxidation of the conductive polymer and lead to a rapid deterioration ofthe electrical properties.

As such, a need exists for an improved solid electrolytic capacitor foruse at high temperature and high levels of humidity.

SUMMARY OF THE INVENTION

In accordance with one embodiment of the present invention, a capacitorassembly is disclosed that comprises a solid electrolytic capacitorelement that contains a sintered porous anode body, a dielectric thatoverlies the anode body, and a solid electrolyte that overlies thedielectric; an anode termination that is in electrical connection withthe anode body; a cathode termination that is in electrical connectionwith the solid electrolyte; and a casing material that encapsulates thecapacitor element and leaves exposed at least a portion of the anodetermination and the cathode termination. The casing material is formedfrom an epoxy composition that contains one or more inorganic oxidefillers and a resinous material that includes one or more epoxy resinscrosslinked with a co-reactant, wherein inorganic oxide fillers about 75wt. % or more of the epoxy composition, and vitreous silica constitutesabout 30 wt. % or more of the total weight of inorganic oxide fillers inthe epoxy composition.

Other features and aspects of the present invention are set forth ingreater detail below.

BRIEF DESCRIPTION OF THE DRAWING

A full and enabling disclosure of the present invention, including thebest mode thereof, directed to one of ordinary skill in the art, is setforth more particularly in the remainder of the specification, whichmakes reference to the appended FIGURE in which:

FIG. 1 is a schematic illustration of one embodiment of a capacitor thatmay be formed in accordance with the present invention.

Repeat use of references characters in the present specification anddrawing is intended to represent same or analogous features or elementsof the invention.

DETAILED DESCRIPTION OF REPRESENTATIVE EMBODIMENTS

It is to be understood by one of ordinary skill in the art that thepresent discussion is a description of exemplary embodiments only, andis not intended as limiting the broader aspects of the presentinvention, which broader aspects are embodied in the exemplaryconstruction.

Generally speaking, the present invention is directed to a capacitorassembly that contains a solid electrolytic capacitor element that isencapsulated within a casing material. The present inventors havediscovered that a capacitor assembly that performs well at hightemperatures and humidity (e.g., 85° C./85% relative humidity) can beachieved in part through selective control over the particular nature ofthe casing material. More particularly, an epoxy composition is used toform the casing that contains one or more inorganic oxide fillers and aresinous material that contains an epoxy resin crosslinked with aco-reactant. To help improve the overall moisture resistance of thecasing, the content of the inorganic oxide fillers is maintained at ahigh level of the composition, such as about 75 wt. % or more. Further,a substantial portion of the fillers is typically in the form ofvitreous silica, which is believed to further improve the moistureresistance of the casing due to its high purity and relatively simplechemical form.

Due to its unique structure, the resulting capacitor is not highlysensitive to moisture and can thus exhibit excellent electricalproperties even when exposed to high humidity levels, such as whenplaced into contact with an atmosphere having a relative humidity ofabout 40% or more, in some embodiments about 45% or more, in someembodiments about 50% or more, and in some embodiments, about 70% ormore (e.g., about 85% to 100%). Relative humidity may, for instance, bedetermined in accordance with ASTM E337-02, Method A (2007). The humidatmosphere may be part of the internal atmosphere of the capacitorassembly itself, or it may be an external atmosphere to which thecapacitor assembly is exposed during storage and/or use. The capacitormay, for instance, exhibit a relatively low equivalence seriesresistance (“ESR”) when exposed to the high humidity atmosphere (e.g.,85% relative humidity), such as about 200 mohms, in some embodimentsless than about 150 mohms, in some embodiments from about 0.01 to about125 mohms, and in some embodiments, from about 0.1 to about 100 mohms,measured at an operating frequency of 100 kHz. The capacitor assemblymay exhibit a DCL of only about 50 microamps (“μA”) or less, in someembodiments about 40 μA or less, in some embodiments about 20 μA orless, and in some embodiments, from about 0.1 to about 10 μA. Thecapacitor assembly may also exhibit a high percentage of its wetcapacitance, which enables it to have only a small capacitance lossand/or fluctuation in the presence of atmosphere humidity. Thisperformance characteristic is quantified by the “wet-to-dry capacitancepercentage”, which is determined by the equation:Wet-to-Dry Capacitance=(Dry Capacitance/Wet Capacitance)×100

The capacitor assembly may exhibit a wet-to-dry capacitance percentageof about 50% or more, in some embodiments about 60% or more, in someembodiments about 70% or more, and in some embodiments, from about 80%to 100%. The dry capacitance may be about 30 nanoFarads per squarecentimeter (“nF/cm²”) or more, in some embodiments about 100 nF/cm² ormore, in some embodiments from about 200 to about 3,000 nF/cm², and insome embodiments, from about 400 to about 2,000 nF/cm², measured at afrequency of 120 Hz.

Notably, the ESR, DCL, and capacitance values may even be maintained fora substantial amount of time and at high temperatures. For example, thevalues may be maintained for about 100 hours or more, in someembodiments from about 300 hours to about 3,000 hours, and in someembodiments, from about 400 hours to about 2,500 hours (e.g., 500 hours,600 hours, 700 hours, 800 hours, 900 hours, 1,000 hours, 1,100 hours,1,200 hours, or 2,000 hours) at temperatures ranging from 50° C. to 250°C., and, in some embodiments from 70° C. to 200° C., and in someembodiments, from 80° C. to about 150° C. (e.g., 85° C.), and at a highhumidity level. In one embodiment, for instance, the values may bemaintained for 1,000 hours at a temperature of 85° C.

Various embodiments of the capacitor will now be described in moredetail.

I. Capacitor Element

A. Anode Body

The capacitor element includes an anode that contains a dielectricformed on a sintered porous body. The porous anode body may be formedfrom a powder that contains a valve metal (i.e., metal that is capableof oxidation) or valve metal-based compound, such as tantalum, niobium,aluminum, hafnium, titanium, alloys thereof, oxides thereof, nitridesthereof, and so forth. The powder is typically formed from a reductionprocess in which a tantalum salt (e.g., potassium fluotantalate(K₂TaF₇), sodium fluotantalate (Na₂TaF₇), tantalum pentachloride(TaCl₅), etc.) is reacted with a reducing agent. The reducing agent maybe provided in the form of a liquid, gas (e.g., hydrogen), or solid,such as a metal (e.g., sodium), metal alloy, or metal salt. In oneembodiment, for instance, a tantalum salt (e.g., TaCl₅) may be heated ata temperature of from about 900° C. to about 2,000° C., in someembodiments from about 1,000° C. to about 1,800° C., and in someembodiments, from about 1,100° C. to about 1,600° C., to form a vaporthat can be reduced in the presence of a gaseous reducing agent (e.g.,hydrogen). Additional details of such a reduction reaction may bedescribed in WO 2014/199480 to Maeshima, et al. After the reduction, theproduct may be cooled, crushed, and washed to form a powder.

The specific charge of the powder typically varies from about 2,000 toabout 800,000 microFarads*Volts per gram (“μF*V/g”) depending on thedesired application. For instance, in certain embodiments, a high chargepowder may be employed that has a specific charge of from about 100,000to about 800,000 μF*V/g, in some embodiments from about 120,000 to about700,000 μF*V/g, and in some embodiments, from about 150,000 to about600,000 μF*V/g. In other embodiments, a low charge powder may beemployed that has a specific charge of from about 2,000 to about 100,000μF*V/g, in some embodiments from about 5,000 to about 80,000 μF*V/g, andin some embodiments, from about 10,000 to about 70,000 μF*V/g. As isknown in the art, the specific charge may be determined by multiplyingcapacitance by the anodizing voltage employed, and then dividing thisproduct by the weight of the anodized electrode body.

The powder may be a free-flowing, finely divided powder that containsprimary particles. The primary particles of the powder generally have amedian size (D50) of from about 5 to about 500 nanometers, in someembodiments from about 10 to about 400 nanometers, and in someembodiments, from about 20 to about 250 nanometers, such as determinedusing a laser particle size distribution analyzer made by BECKMANCOULTER Corporation (e.g., LS-230), optionally after subjecting theparticles to an ultrasonic wave vibration of 70 seconds. The primaryparticles typically have a three-dimensional granular shape (e.g.,nodular or angular). Such particles typically have a relatively low“aspect ratio”, which is the average diameter or width of the particlesdivided by the average thickness (“D/T”). For example, the aspect ratioof the particles may be about 4 or less, in some embodiments about 3 orless, and in some embodiments, from about 1 to about 2. In addition toprimary particles, the powder may also contain other types of particles,such as secondary particles formed by aggregating (or agglomerating) theprimary particles. Such secondary particles may have a median size (D50)of from about 1 to about 500 micrometers, and in some embodiments, fromabout 10 to about 250 micrometers.

Agglomeration of the particles may occur by heating the particles and/orthrough the use of a binder. For example, agglomeration may occur at atemperature of from about 0° C. to about 40° C., in some embodimentsfrom about 5° C. to about 35° C., and in some embodiments, from about15° C. to about 30° C. Suitable binders may likewise include, forinstance, poly(vinyl butyral); poly(vinyl acetate); poly(vinyl alcohol);poly(vinyl pyrollidone); cellulosic polymers, such ascarboxymethylcellulose, methyl cellulose, ethyl cellulose, hydroxyethylcellulose, and methylhydroxyethyl cellulose; atactic polypropylene,polyethylene; polyethylene glycol (e.g., Carbowax from Dow ChemicalCo.); polystyrene, poly(butadiene/styrene); polyamides, polyimides, andpolyacrylamides, high molecular weight polyethers; copolymers ofethylene oxide and propylene oxide; fluoropolymers, such aspolytetrafluoroethylene, polyvinylidene fluoride, and fluoro-olefincopolymers; acrylic polymers, such as sodium polyacrylate, poly(loweralkyl acrylates), poly(lower alkyl methacrylates) and copolymers oflower alkyl acrylates and methacrylates; and fatty acids and waxes, suchas stearic and other soapy fatty acids, vegetable wax, microwaxes(purified paraffins), etc.

The resulting powder may be compacted to form a pellet using anyconventional powder press device. For example, a press mold may beemployed that is a single station compaction press containing a die andone or multiple punches. Alternatively, anvil-type compaction pressmolds may be used that use only a die and single lower punch. Singlestation compaction press molds are available in several basic types,such as cam, toggle/knuckle and eccentric/crank presses with varyingcapabilities, such as single action, double action, floating die,movable platen, opposed ram, screw, impact, hot pressing, coining orsizing. The powder may be compacted around an anode lead, which may bein the form of a wire, sheet, etc. The lead may extend in a longitudinaldirection from the anode body and may be formed from any electricallyconductive material, such as tantalum, niobium, aluminum, hafnium,titanium, etc., as well as electrically conductive oxides and/ornitrides of thereof. Connection of the lead may also be accomplishedusing other known techniques, such as by welding the lead to the body orembedding it within the anode body during formation (e.g., prior tocompaction and/or sintering).

Any binder may be removed after pressing by heating the pellet undervacuum at a certain temperature (e.g., from about 150° C. to about 500°C.) for several minutes. Alternatively, the binder may also be removedby contacting the pellet with an aqueous solution, such as described inU.S. Pat. No. 6,197,252 to Bishop, et al. Thereafter, the pellet issintered to form a porous, integral mass. The pellet is typicallysintered at a temperature of from about 700° C. to about 1600° C., insome embodiments from about 800° C. to about 1500° C., and in someembodiments, from about 900° C. to about 1200° C., for a time of fromabout 5 minutes to about 100 minutes, and in some embodiments, fromabout 8 minutes to about 15 minutes. This may occur in one or moresteps. If desired, sintering may occur in an atmosphere that limits thetransfer of oxygen atoms to the anode. For example, sintering may occurin a reducing atmosphere, such as in a vacuum, inert gas, hydrogen, etc.The reducing atmosphere may be at a pressure of from about 10 Torr toabout 2000 Torr, in some embodiments from about 100 Torr to about 1000Torr, and in some embodiments, from about 100 Torr to about 930 Torr.Mixtures of hydrogen and other gases (e.g., argon or nitrogen) may alsobe employed.

B. Dielectric

The anode is also coated with a dielectric. The dielectric may be formedby anodically oxidizing (“anodizing”) the sintered anode so that adielectric layer is formed over and/or within the anode. For example, atantalum (Ta) anode may be anodized to tantalum pentoxide (Ta₂O₅).Typically, anodization is performed by initially applying a solution tothe anode, such as by dipping anode into the electrolyte. A solvent isgenerally employed, such as water (e.g., deionized water). To enhanceionic conductivity, a compound may be employed that is capable ofdissociating in the solvent to form ions. Examples of such compoundsinclude, for instance, acids, such as described below with respect tothe electrolyte. For example, an acid (e.g., phosphoric acid) mayconstitute from about 0.01 wt. % to about 5 wt. %, in some embodimentsfrom about 0.05 wt. % to about 0.8 wt. %, and in some embodiments, fromabout 0.1 wt. % to about 0.5 wt. % of the anodizing solution. Ifdesired, blends of acids may also be employed.

A current is passed through the anodizing solution to form thedielectric layer. The value of the formation voltage manages thethickness of the dielectric layer. For example, the power supply may beinitially set up at a galvanostatic mode until the required voltage isreached. Thereafter, the power supply may be switched to apotentiostatic mode to ensure that the desired dielectric thickness isformed over the entire surface of the anode. Of course, other knownmethods may also be employed, such as pulse or step potentiostaticmethods. The voltage at which anodic oxidation occurs typically rangesfrom about 4 to about 250 V, and in some embodiments, from about 5 toabout 200 V, and in some embodiments, from about 10 to about 150 V.During oxidation, the anodizing solution can be kept at an elevatedtemperature, such as about 30° C. or more, in some embodiments fromabout 40° C. to about 200° C., and in some embodiments, from about 50°C. to about 100° C. Anodic oxidation can also be done at ambienttemperature or lower. The resulting dielectric layer may be formed on asurface of the anode and within its pores.

Although not required, in certain embodiments, the dielectric layer maypossess a differential thickness throughout the anode in that itpossesses a first portion that overlies an external surface of the anodeand a second portion that overlies an interior surface of the anode. Insuch embodiments, the first portion is selectively formed so that itsthickness is greater than that of the second portion. It should beunderstood, however, that the thickness of the dielectric layer need notbe uniform within a particular region. Certain portions of thedielectric layer adjacent to the external surface may, for example,actually be thinner than certain portions of the layer at the interiorsurface, and vice versa. Nevertheless, the dielectric layer may beformed such that at least a portion of the layer at the external surfacehas a greater thickness than at least a portion at the interior surface.Although the exact difference in these thicknesses may vary depending onthe particular application, the ratio of the thickness of the firstportion to the thickness of the second portion is typically from about1.2 to about 40, in some embodiments from about 1.5 to about 25, and insome embodiments, from about 2 to about 20.

To form a dielectric layer having a differential thickness, amulti-stage process is generally employed. In each stage of the process,the sintered anode is anodically oxidized (“anodized”) to form adielectric layer (e.g., tantalum pentoxide). During the first stage ofanodization, a relatively small forming voltage is typically employed toensure that the desired dielectric thickness is achieved for the innerregion, such as forming voltages ranging from about 1 to about 90 volts,in some embodiments from about 2 to about 50 volts, and in someembodiments, from about 5 to about 20 volts. Thereafter, the sinteredbody may then be anodically oxidized in a second stage of the process toincrease the thickness of the dielectric to the desired level. This isgenerally accomplished by anodizing in an electrolyte at a highervoltage than employed during the first stage, such as at formingvoltages ranging from about 50 to about 350 volts, in some embodimentsfrom about 60 to about 300 volts, and in some embodiments, from about 70to about 200 volts. During the first and/or second stages, theelectrolyte may be kept at a temperature within the range of from about15° C. to about 95° C., in some embodiments from about 20° C. to about90° C., and in some embodiments, from about 25° C. to about 85° C.

The electrolytes employed during the first and second stages of theanodization process may be the same or different. Typically, however, itis desired to employ different solutions to help better facilitate theattainment of a higher thickness at the outer portions of the dielectriclayer. For example, it may be desired that the electrolyte employed inthe second stage has a lower ionic conductivity than the electrolyteemployed in the first stage to prevent a significant amount of oxidefilm from forming on the internal surface of anode. In this regard, theelectrolyte employed during the first stage may contain an acidiccompound, such as hydrochloric acid, nitric acid, sulfuric acid,phosphoric acid, polyphosphoric acid, boric acid, boronic acid, etc.Such an electrolyte may have an electrical conductivity of from about0.1 to about 100 mS/cm, in some embodiments from about 0.2 to about 20mS/cm, and in some embodiments, from about 1 to about 10 mS/cm,determined at a temperature of 25° C. The electrolyte employed duringthe second stage typically contains a salt of a weak acid so that thehydronium ion concentration increases in the pores as a result of chargepassage therein. Ion transport or diffusion is such that the weak acidanion moves into the pores as necessary to balance the electricalcharges. As a result, the concentration of the principal conductingspecies (hydronium ion) is reduced in the establishment of equilibriumbetween the hydronium ion, acid anion, and undissociated acid, thusforms a poorer-conducting species. The reduction in the concentration ofthe conducting species results in a relatively high voltage drop in theelectrolyte, which hinders further anodization in the interior while athicker oxide layer, is being built up on the outside to a higherformation voltage in the region of continued high conductivity. Suitableweak acid salts may include, for instance, ammonium or alkali metalsalts (e.g., sodium, potassium, etc.) of boric acid, boronic acid,acetic acid, oxalic acid, lactic acid, adipic acid, etc. Particularlysuitable salts include sodium tetraborate and ammonium pentaborate. Suchelectrolytes typically have an electrical conductivity of from about 0.1to about 20 mS/cm, in some embodiments from about 0.5 to about 10 mS/cm,and in some embodiments, from about 1 to about 5 mS/cm, determined at atemperature of 25° C.

If desired, each stage of anodization may be repeated for one or morecycles to achieve the desired dielectric thickness. Furthermore, theanode may also be rinsed or washed with another solvent (e.g., water)after the first and/or second stages to remove the electrolyte.

C. Solid Electrolyte

As indicated above, a solid electrolyte overlies the dielectric andgenerally functions as the cathode for the capacitor assembly. The solidelectrolyte may include materials as is known in the art, such asconductive polymers (e.g., polypyrroles, polythiophenes, polyanilines,etc.), manganese dioxide, and so forth. Typically, however, the solidelectrolyte contains one or more layers containing extrinsically and/orintrinsically conductive polymer particles. One benefit of employingsuch particles is that they can minimize the presence of ionic species(e.g., Fe²⁺ or Fe³⁺) produced during conventional in situ polymerizationprocesses, which can cause dielectric breakdown under high electricfield due to ionic migration. Thus, by applying the conductive polymeras pre-polymerized particles rather through in situ polymerization, theresulting capacitor may exhibit a relatively high “breakdown voltage.”If desired, the solid electrolyte may be formed from one or multiplelayers. When multiple layers are employed, it is possible that one ormore of the layers includes a conductive polymer formed by in situpolymerization. However, when it is desired to achieve very highbreakdown voltages, the present inventors have discovered that the solidelectrolyte is formed primarily from the conductive particles describedabove, and that it is generally free of conductive polymers formed viain situ polymerization. Regardless of the number of layers employed, theresulting solid electrolyte typically has a total a thickness of fromabout 1 micrometer (μm) to about 200 μm, in some embodiments from about2 μm to about 50 μm, and in some embodiments, from about 5 μm to about30 μm.

Thiophene polymers are particularly suitable for use in the solidelectrolyte. In certain embodiments, for instance, an “extrinsically”conductive thiophene polymer may be employed in the solid electrolytethat has repeating units of the following formula (III):

wherein,

R₇ is a linear or branched, C₁ to C₁₈ alkyl radical (e.g., methyl,ethyl, n- or iso-propyl, n-, iso-, sec- or tert-butyl, n-pentyl,1-methylbutyl, 2-methylbutyl, 3-methylbutyl, 1-ethylpropyl,1,1-dimethylpropyl, 1,2-dimethylpropyl, 2,2-dimethylpropyl, n-hexyl,n-heptyl, n-octyl, 2-ethylhexyl, n-nonyl, n-decyl, n-undecyl, n-dodecyl,n-tridecyl, n-tetradecyl, n-hexadecyl, n-octadecyl, etc.); C₅ to C₁₂cycloalkyl radical (e.g., cyclopentyl, cyclohexyl, cycloheptyl,cyclooctyl, cyclononyl, cyclodecyl, etc.); C₆ to C₁₄ aryl radical (e.g.,phenyl, naphthyl, etc.); C₇ to C₁₈ aralkyl radical (e.g., benzyl, o-,m-, p-tolyl, 2,3-, 2,4-, 2,5-, 2-6, 3-4-, 3,5-xylyl, mesityl, etc.); and

q is an integer from 0 to 8, in some embodiments, from 0 to 2, and inone embodiment, 0. In one particular embodiment, “q” is 0 and thepolymer is poly(3,4-ethylenedioxythiophene). One commercially suitableexample of a monomer suitable for forming such a polymer is3,4-ethylenedioxthiophene, which is available from Heraeus under thedesignation Clevios™ M.

The polymers of formula (III) are generally considered to be“extrinsically” conductive to the extent that they typically require thepresence of a separate counterion that is not covalently bound to thepolymer. The counterion may be a monomeric or polymeric anion thatcounteracts the charge of the conductive polymer. Polymeric anions can,for example, be anions of polymeric carboxylic acids (e.g., polyacrylicacids, polymethacrylic acid, polymaleic acids, etc.); polymeric sulfonicacids (e.g., polystyrene sulfonic acids (“PSS”), polyvinyl sulfonicacids, etc.); and so forth. The acids may also be copolymers, such ascopolymers of vinyl carboxylic and vinyl sulfonic acids with otherpolymerizable monomers, such as acrylic acid esters and styrene.Likewise, suitable monomeric anions include, for example, anions of C₁to C₂₀ alkane sulfonic acids (e.g., dodecane sulfonic acid); aliphaticperfluorosulfonic acids (e.g., trifluoromethane sulfonic acid,perfluorobutane sulfonic acid or perfluorooctane sulfonic acid);aliphatic C₁ to C₂₀ carboxylic acids (e.g., 2-ethyl-hexylcarboxylicacid); aliphatic perfluorocarboxylic acids (e.g., trifluoroacetic acidor perfluorooctanoic acid); aromatic sulfonic acids optionallysubstituted by C₁ to C₂₀ alkyl groups (e.g., benzene sulfonic acid,o-toluene sulfonic acid, p-toluene sulfonic acid or dodecylbenzenesulfonic acid); cycloalkane sulfonic acids (e.g., camphor sulfonic acidor tetrafluoroborates, hexafluorophosphates, perchlorates,hexafluoroantimonates, hexafluoroarsenates or hexachloroantimonates);and so forth. Particularly suitable counteranions are polymeric anions,such as a polymeric carboxylic or sulfonic acid (e.g., polystyrenesulfonic acid (“PSS”)). The molecular weight of such polymeric anionstypically ranges from about 1,000 to about 2,000,000, and in someembodiments, from about 2,000 to about 500,000.

Intrinsically conductive polymers may also be employed that have apositive charge located on the main chain that is at least partiallycompensated by anions covalently bound to the polymer. For example, oneexample of a suitable intrinsically conductive thiophene polymer mayhave repeating units of the following formula (IV):

wherein,

R is (CH₂)_(a)—O—(CH₂)_(b);

a is from 0 to 10, in some embodiments from 0 to 6, and in someembodiments, from 1 to 4 (e.g., 1);

b is from 1 to 18, in some embodiments from 1 to 10, and in someembodiments, from 2 to 6 (e.g., 2, 3, 4, or 5);

Z is an anion, such as SO₃ ⁻, C(O)O⁻, BF₄ ⁻, CF₃SO₃ ⁻, SbF₆ ⁻,N(SO₂CF₃)₂ ⁻, C₄H₃O₄ ⁻, ClO₄ ⁻, etc.;

X is a cation, such as hydrogen, an alkali metal (e.g., lithium, sodium,rubidium, cesium or potassium), ammonium, etc.

In one particular embodiment, Z in formula (IV) is a sulfonate ion suchthat the intrinsically conductive polymer contains repeating units ofthe following formula (V):

wherein, R and X are defined above. In formula (IV) or (V), a ispreferably 1 and b is preferably 3 or 4. Likewise, X is preferablysodium or potassium.

If desired, the polymer may be a copolymer that contains other types ofrepeating units. In such embodiments, the repeating units of formula(IV) typically constitute about 50 mol. % or more, in some embodimentsfrom about 75 mol. % to about 99 mol. %, and in some embodiments, fromabout 85 mol. % to about 95 mol. % of the total amount of repeatingunits in the copolymer. Of course, the polymer may also be a homopolymerto the extent that it contains 100 mol. % of the repeating units offormula (IV). Specific examples of such homopolymers includepoly(4-(2,3-dihydrothieno-[3,4-b][1,4]dioxin-2-ylmethoxy)-1-butane-sulphonicacid, salt) andpoly(4-(2,3-dihydrothieno-[3,4-b][1,4]dioxin-2-ylmethoxy)-I-propanesulphonicacid, salt).

Regardless of the particular nature of the polymer, the resultingconductive polymer particles typically have an average size (e.g.,diameter) of from about 1 to about 80 nanometers, in some embodimentsfrom about 2 to about 70 nanometers, and in some embodiments, from about3 to about 60 nanometers. The diameter of the particles may bedetermined using known techniques, such as by ultracentrifuge, laserdiffraction, etc. The shape of the particles may likewise vary. In oneparticular embodiment, for instance, the particles are spherical inshape. However, it should be understood that other shapes are alsocontemplated by the present invention, such as plates, rods, discs,bars, tubes, irregular shapes, etc.

Although not necessarily required, the conductive polymer particles maybe applied in the form of a dispersion. The concentration of theconductive polymer in the dispersion may vary depending on the desiredviscosity of the dispersion and the particular manner in which thedispersion is to be applied to the capacitor element. Typically,however, the polymer constitutes from about 0.1 to about 10 wt. %, insome embodiments from about 0.4 to about 5 wt. %, and in someembodiments, from about 0.5 to about 4 wt. % of the dispersion. Thedispersion may also contain one or more components to enhance theoverall properties of the resulting solid electrolyte. For example, thedispersion may contain a binder to further enhance the adhesive natureof the polymeric layer and also increase the stability of the particleswithin the dispersion. The binder may be organic in nature, such aspolyvinyl alcohols, polyvinyl pyrrolidones, polyvinyl chlorides,polyvinyl acetates, polyvinyl butyrates, polyacrylic acid esters,polyacrylic acid amides, polymethacrylic acid esters, polymethacrylicacid amides, polyacrylonitriles, styrene/acrylic acid ester, vinylacetate/acrylic acid ester and ethylene/vinyl acetate copolymers,polybutadienes, polyisoprenes, polystyrenes, polyethers, polyesters,polycarbonates, polyurethanes, polyamides, polyimides, polysulfones,melamine formaldehyde resins, epoxide resins, silicone resins orcelluloses. Crosslinking agents may also be employed to enhance theadhesion capacity of the binders. Such crosslinking agents may include,for instance, melamine compounds, masked isocyanates or crosslinkablepolymers, such as polyurethanes, polyacrylates or polyolefins, andsubsequent crosslinking. Dispersion agents may also be employed tofacilitate the ability to apply the layer to the anode. Suitabledispersion agents include solvents, such as aliphatic alcohols (e.g.,methanol, ethanol, i-propanol and butanol), aliphatic ketones (e.g.,acetone and methyl ethyl ketones), aliphatic carboxylic acid esters(e.g., ethyl acetate and butyl acetate), aromatic hydrocarbons (e.g.,toluene and xylene), aliphatic hydrocarbons (e.g., hexane, heptane andcyclohexane), chlorinated hydrocarbons (e.g., dichloromethane anddichloroethane), aliphatic nitriles (e.g., acetonitrile), aliphaticsulfoxides and sulfones (e.g., dimethyl sulfoxide and sulfolane),aliphatic carboxylic acid amides (e.g., methylacetamide,dimethylacetamide and dimethylformamide), aliphatic and araliphaticethers (e.g., diethylether and anisole), water, and mixtures of any ofthe foregoing solvents. A particularly suitable dispersion agent iswater.

In addition to those mentioned above, still other ingredients may alsobe used in the dispersion. For example, conventional fillers may be usedthat have a size of from about 10 nanometers to about 100 micrometers,in some embodiments from about 50 nanometers to about 50 micrometers,and in some embodiments, from about 100 nanometers to about 30micrometers. Examples of such fillers include calcium carbonate,silicates, silica, calcium or barium sulfate, aluminum hydroxide, glassfibers or bulbs, wood flour, cellulose powder carbon black, electricallyconductive polymers, etc. The fillers may be introduced into thedispersion in powder form, but may also be present in another form, suchas fibers.

Surface-active substances may also be employed in the dispersion, suchas ionic or non-ionic surfactants. Furthermore, adhesives may beemployed, such as organofunctional silanes or their hydrolysates, forexample 3-glycidoxypropyltrialkoxysilane, 3-aminopropyl-triethoxysilane,3-mercaptopropyl -trimethoxysilane,3-metacryloxypropyltrimethoxysilane,vinyltrimethoxysilane or octyltriethoxysilane. The dispersion may alsocontain additives that increase conductivity, such as ethergroup-containing compounds (e.g., tetrahydrofuran), lactonegroup-containing compounds (e.g., γ-butyrolactone or γ-valerolactone),amide or lactam group-containing compounds (e.g., caprolactam,N-methylcaprolactam, N,N-dimethylacetamide, N-methylacetamide,N,N-dimethylformamide (DMF), N-methylformamide, N-methylformanilide,N-methylpyrrolidone (NMP), N-octylpyrrolidone, or pyrrolidone), sulfonesand sulfoxides (e.g., sulfolane (tetramethylenesulfone) ordimethylsulfoxide (DMSO)), sugar or sugar derivatives (e.g., saccharose,glucose, fructose, or lactose), sugar alcohols (e.g., sorbitol ormannitol), furan derivatives (e.g., 2-furancarboxylic acid or3-furancarboxylic acid), an alcohols (e.g., ethylene glycol, glycerol,di- or triethylene glycol).

The dispersion may be applied using a variety of known techniques, suchas by spin coating, impregnation, pouring, dropwise application,injection, spraying, doctor blading, brushing, printing (e.g., ink-jet,screen, or pad printing), or dipping. The viscosity of the dispersion istypically from about 0.1 to about 100,000 mPas (measured at a shear rateof 100 s⁻¹), in some embodiments from about 1 to about 10,000 mPas, insome embodiments from about 10 to about 1,500 mPas, and in someembodiments, from about 100 to about 1000 mPas.

i. Inner Layers

The solid electrolyte is generally formed from one or more “inner”conductive polymer layers. The term “inner” in this context refers toone or more layers that overly the dielectric, whether directly or viaanother layer (e.g., pre-coat layer). One or multiple inner layers maybe employed. For example, the solid electrolyte typically contains from2 to 30, in some embodiments from 4 to 20, and in some embodiments, fromabout 5 to 15 inner layers (e.g., 10 layers). The inner layer(s) may,for example, contain intrinsically and/or extrinsically conductivepolymer particles such as described above. For instance, such particlesmay constitute about 50 wt. % or more, in some embodiments about 70 wt.% or more, and in some embodiments, about 90 wt. % or more (e.g., 100wt. %) of the inner layer(s). In alternative embodiments, the innerlayer(s) may contain an in-situ polymerized conductive polymer. In suchembodiments, the in-situ polymerized polymers may constitute about 50wt. % or more, in some embodiments about 70 wt. % or more, and in someembodiments, about 90 wt. % or more (e.g., 100 wt. %) of the innerlayer(s).

ii. Outer Layers

The solid electrolyte may also contain one or more optional “outer”conductive polymer layers that overly the inner layer(s) and are formedfrom a different material. For example, the outer layer(s) may containextrinsically conductive polymer particles. In one particularembodiment, the outer layer(s) are formed primarily from suchextrinsically conductive polymer particles in that they constitute about50 wt. % or more, in some embodiments about 70 wt. % or more, and insome embodiments, about 90 wt. % or more (e.g., 100 wt. %) of arespective outer layer. One or multiple outer layers may be employed.For example, the solid electrolyte may contain from 2 to 30, in someembodiments from 4 to 20, and in some embodiments, from about 5 to 15outer layers, each of which may optionally be formed from a dispersionof the extrinsically conductive polymer particles.

D. External Polymer Coating

An external polymer coating may also overly the solid electrolyte. Theexternal polymer coating generally contains one or more layers formedfrom pre-polymerized conductive polymer particles such as describedabove (e.g., dispersion of extrinsically conductive polymer particles).The external coating may be able to further penetrate into the edgeregion of the capacitor body to increase the adhesion to the dielectricand result in a more mechanically robust part, which may reduceequivalent series resistance and leakage current. Because it isgenerally intended to improve the degree of edge coverage rather toimpregnate the interior of the anode body, the particles used in theexternal coating typically have a larger size than those employed in thesolid electrolyte. For example, the ratio of the average size of theparticles employed in the external polymer coating to the average sizeof the particles employed in any dispersion of the solid electrolyte istypically from about 1.5 to about 30, in some embodiments from about 2to about 20, and in some embodiments, from about 5 to about 15. Forexample, the particles employed in the dispersion of the externalcoating may have an average size of from about 80 to about 500nanometers, in some embodiments from about 90 to about 250 nanometers,and in some embodiments, from about 100 to about 200 nanometers.

If desired, a crosslinking agent may also be employed in the externalpolymer coating to enhance the degree of adhesion to the solidelectrolyte. Typically, the crosslinking agent is applied prior toapplication of the dispersion used in the external coating. Suitablecrosslinking agents are described, for instance, in U.S. PatentPublication No. 2007/0064376 to Merker, et al. and include, forinstance, amines (e.g., diamines, triamines, oligomer amines,polyamines, etc.); polyvalent metal cations, such as salts or compoundsof Mg, Al, Ca, Fe, Cr, Mn, Ba, Ti, Co, Ni, Cu, Ru, Ce or Zn, phosphoniumcompounds, sulfonium compounds, etc. Particularly suitable examplesinclude, for instance, 1,4-diaminocyclohexane,1,4-bis(amino-methyl)cyclohexane, ethylenediamine, 1,6-hexanediamine,1,7-heptanediamine, 1,8-octanediamine, 1,9-nonanediamine,1,10-decanediamine, 1,12-dodecanediamine, N,N-dimethylethylenediamine,N,N,N′,N′-tetramethylethylenediamine,N,N,N′,N′-tetramethyl-1,4-butanediamine, etc., as well as mixturesthereof.

The crosslinking agent is typically applied from a solution ordispersion whose pH is from 1 to 10, in some embodiments from 2 to 7, insome embodiments, from 3 to 6, as determined at 25° C. Acidic compoundsmay be employed to help achieve the desired pH level. Examples ofsolvents or dispersants for the crosslinking agent include water ororganic solvents, such as alcohols, ketones, carboxylic esters, etc. Thecrosslinking agent may be applied to the capacitor body by any knownprocess, such as spin-coating, impregnation, casting, dropwiseapplication, spray application, vapor deposition, sputtering,sublimation, knife-coating, painting or printing, for example inkjet,screen or pad printing. Once applied, the crosslinking agent may bedried prior to application of the polymer dispersion. This process maythen be repeated until the desired thickness is achieved. For example,the total thickness of the entire external polymer coating, includingthe crosslinking agent and dispersion layers, may range from about 1 toabout 50 μm, in some embodiments from about 2 to about 40 μm, and insome embodiments, from about 5 to about 20 μm.

E. Cathode Coating

If desired, the capacitor element may also employ a cathode coating thatoverlies the solid electrolyte and other optional layers (e.g., externalpolymer coating). The cathode coating may contain a metal particle layerincludes a plurality of conductive metal particles dispersed within aresinous polymer matrix. The particles typically constitute from about50 wt. % to about 99 wt. %, in some embodiments from about 60 wt. % toabout 98 wt. %, and in some embodiments, from about 70 wt. % to about 95wt. % of the layer, while the resinous polymer matrix typicallyconstitutes from about 1 wt. % to about 50 wt. %, in some embodimentsfrom about 2 wt. % to about 40 wt. %, and in some embodiments, fromabout 5 wt. % to about 30 wt. % of the layer.

The conductive metal particles may be formed from a variety of differentmetals, such as copper, nickel, silver, nickel, zinc, tin, lead, copper,aluminum, molybdenum, titanium, iron, zirconium, magnesium, etc., aswell as alloys thereof. Silver is a particularly suitable conductivemetal for use in the layer. The metal particles often have a relativelysmall size, such as an average size of from about 0.01 to about 50micrometers, in some embodiments from about 0.1 to about 40 micrometers,and in some embodiments, from about 1 to about 30 micrometers.Typically, only one metal particle layer is employed, although it shouldbe understood that multiple layers may be employed if so desired. Thetotal thickness of such layer(s) is typically within the range of fromabout 1 μm to about 500 μm, in some embodiments from about 5 μm to about200 μm, and in some embodiments, from about 10 μm to about 100 μm.

The resinous polymer matrix typically includes a polymer, which may bethermoplastic or thermosetting in nature. Typically, however, thepolymer is selected so that it can act as a barrier to electromigrationof silver ions, and also so that it contains a relatively small amountof polar groups to minimize the degree of water adsorption in thecathode coating. In this regard, the present inventors have found thatvinyl acetal polymers are particularly suitable for this purpose, suchas polyvinyl butyral, polyvinyl formal, etc. Polyvinyl butyral, forinstance, may be formed by reacting polyvinyl alcohol with an aldehyde(e.g., butyraldehyde). Because this reaction is not typically complete,polyvinyl butyral will generally have a residual hydroxyl content. Byminimizing this content, however, the polymer can possess a lesserdegree of strong polar groups, which would otherwise result in a highdegree of moisture adsorption and result in silver ion migration. Forinstance, the residual hydroxyl content in polyvinyl acetal may be about35 mol. % or less, in some embodiments about 30 mol. % or less, and insome embodiments, from about 10 mol. % to about 25 mol. %. Onecommercially available example of such a polymer is available fromSekisui Chemical Co., Ltd. under the designation “BH-S” (polyvinylbutyral).

To form the cathode coating, a conductive paste is typically applied tothe capacitor that overlies the solid electrolyte. One or more organicsolvents are generally employed in the paste. A variety of differentorganic solvents may generally be employed, such as glycols (e.g.,propylene glycol, butylene glycol, triethylene glycol, hexylene glycol,polyethylene glycols, ethoxydiglycol, and dipropyleneglycol); glycolethers (e.g., methyl glycol ether, ethyl glycol ether, and isopropylglycol ether); ethers (e.g., diethyl ether and tetrahydrofuran);alcohols (e.g., benzyl alcohol, methanol, ethanol, n-propanol,iso-propanol, and butanol); triglycerides; ketones (e.g., acetone,methyl ethyl ketone, and methyl isobutyl ketone); esters (e.g., ethylacetate, butyl acetate, diethylene glycol ether acetate, andmethoxypropyl acetate); amides (e.g., dimethylformamide,dimethylacetamide, dimethylcaprylic/capric fatty acid amide andN-alkylpyrrolidones); nitriles (e.g., acetonitrile, propionitrile,butyronitrile and benzonitrile); sulfoxides or sulfones (e.g., dimethylsulfoxide (DMSO) and sulfolane); etc., as well as mixtures thereof. Theorganic solvent(s) typically constitute from about 10 wt. % to about 70wt. %, in some embodiments from about 20 wt. % to about 65 wt. %, and insome embodiments, from about 30 wt. % to about 60 wt. %. of the paste.Typically, the metal particles constitute from about 10 wt. % to about60 wt. %, in some embodiments from about 20 wt. % to about 45 wt. %, andin some embodiments, from about 25 wt. % to about 40 wt. % of the paste,and the resinous polymer matrix constitutes from about 0.1 wt. % toabout 20 wt. %, in some embodiments from about 0.2 wt. % to about 10 wt.%, and in some embodiments, from about 0.5 wt. % to about 8 wt. % of thepaste.

The paste may have a relatively low viscosity, allowing it to be readilyhandled and applied to a capacitor element. The viscosity may, forinstance, range from about 50 to about 3,000 centipoise, in someembodiments from about 100 to about 2,000 centipoise, and in someembodiments, from about 200 to about 1,000 centipoise, such as measuredwith a Brookfield DV-1 viscometer (cone and plate) operating at a speedof 10 rpm and a temperature of 25° C. If desired, thickeners or otherviscosity modifiers may be employed in the paste to increase or decreaseviscosity. Further, the thickness of the applied paste may also berelatively thin and still achieve the desired properties. For example,the thickness of the paste may be from about 0.01 to about 50micrometers, in some embodiments from about 0.5 to about 30 micrometers,and in some embodiments, from about 1 to about 25 micrometers. Onceapplied, the metal paste may be optionally dried to remove certaincomponents, such as the organic solvents. For instance, drying may occurat a temperature of from about 20° C. to about 150° C., in someembodiments from about 50° C. to about 140° C., and in some embodiments,from about 80° C. to about 130° C.

F. Other Components

If desired, the capacitor may also contain other layers as is known inthe art. In certain embodiments, for instance, a carbon layer (e.g.,graphite) may be positioned between the solid electrolyte and the silverlayer that can help further limit contact of the silver layer with thesolid electrolyte.

In addition, a pre-coat layer may be employed in certain embodimentsthat overlies the dielectric and includes an organometallic compound.The organometallic compound may have the following general formula:

wherein,

M is an organometallic atom, such as silicon, titanium, and so forth;

R₁, R₂, and R₃ are independently an alkyl (e.g., methyl, ethyl, propyl,etc.) or a hydroxyalkyl (e.g., hydroxymethyl, hydroxyethyl,hydroxypropyl, etc.), wherein at least one of R₁, R₂, and R₃ is ahydroxyalkyl;

n is an integer from 0 to 8, in some embodiments from 1 to 6, and insome embodiments, from 2 to 4 (e.g., 3); and

X is an organic or inorganic functional group, such as glycidyl,glycidyloxy, mercapto, amino, vinyl, etc.

In certain embodiments, R₁, R₂, and R₃ may a hydroxyalkyl (e.g., OCH₃).In other embodiments, however, R₁ may be an alkyl (e.g., CH₃) and R₂ andR₃ may a hydroxyalkyl (e.g., OCH₃).

Further, in certain embodiments, M may be silicon so that theorganometallic compound is an organosilane compound, such as analkoxysilane. Suitable alkoxysilanes may include, for instance,3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane,3-aminopropylmethyldimethoxysilane, 3-aminopropylmethyldiethoxysilane,3-(2-aminoethyl)aminopropyltrimethoxysilane,3-mercaptopropyltrimethoxysilane, 3-mercaptopropyltriethoxysilane,3-mercaptopropylmethyldimethoxysilane,3-mercaptopropylmethyldiethoxysilane, glycidoxymethyltrimethoxysilane,glycidoxymethyltriethoxysilane, glycidoxymethyl-tripropoxysilane,glycidoxymethyltributoxysilane, β-glycidoxyethyltrimethoxysilane,β-glycidoxyethyltriethoxysilane, β-glycidoxyethyl-tripropoxysilane,β-glycidoxyethyl-tributoxysilane, β-glycidoxyethyltrimethoxysilane,α-glycidoxyethyltriethoxysilane, α-glycidoxyethyltripropoxysilane,α-glycidoxyethyltributoxysilane, γ-glycidoxypropyl-trimethoxysilane,γ-glycidoxypropyltriethoxysilane, γ-glycidoxypropyl-tripropoxysilane,γ-glycidoxypropyltributoxysilane, β-glycidoxypropyltrimethoxysilane,β-glycidoxypropyl-triethoxysilane, β-glycidoxypropyltripropoxysilane,α-glycidoxypropyltributoxysilane, α-glycidoxypropyltrimethoxysilane,α-glycidoxypropyltriethoxysilane, α-glycidoxypropyl-tripropoxysilane,α-glycidoxypropyltributoxysilane, γ-glycidoxybutyltrimethoxysilane,δ-glycidoxybutyltriethoxysilane, δ-glycidoxybutyltripropoxysilane,δ-glycidoxybutyl-tributoxysilane, δ-glycidoxybutyltrimethoxysilane,γ-glycidoxybutyltriethoxysilane, γ-glycidoxybutyltripropoxysilane,γ-propoxybutyltributoxysilane, δ-glycidoxybutyl-trimethoxysilane,δ-glycidoxybutyltriethoxysilane, δ-glycidoxybutyltripropoxysilane,α-glycidoxybutyltrimethoxysilane, α-glycidoxybutyltriethoxysilane,α-glycidoxybutyl-tripropoxysilane, α-glycidoxybutyltributoxysilane,(3,4-epoxycyclohexyl)-methyl-trimethoxysilane,(3,4-epoxycyclohexyl)methyl-triethoxysilane,(3,4-epoxycyclohexyl)methyltripropoxysilane,(3,4-epoxycyclohexyl)-methyl-tributoxysilane,(3,4-epoxycyclohexyl)ethyl-trimethoxysilane,(3,4-epoxycyclohexyl)ethyl-triethoxysilane,(3,4-epoxycyclohexyl)ethyltripropoxysilane,(3,4-epoxycyclohexyl)ethyltributoxysilane,(3,4-epoxycyclohexyl)propyltrimethoxysilane,(3,4-epoxycyclohexyl)propyltriethoxysilane,(3,4-epoxycyclohexyl)propyl-tripropoxysilane,(3,4-epoxycyclohexyl)propyltributoxysilane,(3,4-epoxycyclohexyl)butyltrimethoxysilane, (3,4-epoxycyclohexy)butyltriethoxysilane, (3,4-epoxycyclohexyl)butyltripropoxysilane,(3,4-epoxycyclohexyl)butyltributoxysilane, and so forth.

The particular manner in which the pre-coat layer is applied to thecapacitor body may vary as desired. In one particular embodiment, thecompound is dissolved in an organic solvent and applied to the part as asolution, such as by screen-printing, dipping, electrophoretic coating,spraying, etc. The organic solvent may vary, but is typically analcohol, such as methanol, ethanol, etc. Organometallic compounds mayconstitute from about 0.1 wt. % to about 10 wt. %, in some embodimentsfrom about 0.2 wt. % to about 8 wt. %, and in some embodiments, fromabout 0.5 wt. % to about 5 wt. % of the solution. Solvents may likewiseconstitute from about 90 wt. % to about 99.9 wt. %, in some embodimentsfrom about 92 wt. % to about 99.8 wt. %, and in some embodiments, fromabout 95 wt. % to about 99.5 wt. % of the solution. Once applied, thepart may then be dried to remove the solvent therefrom and form apre-coat layer containing the organometallic compound.

II. Terminations

Once the desired layers are formed, the capacitor assembly may beprovided with terminations. For example, the capacitor assembly maycontain an anode termination to which an anode lead of the capacitorelement is electrically connected and a cathode termination to which thecathode of the capacitor element is electrically connected. Anyconductive material may be employed to form the terminations, such as aconductive metal (e.g., copper, nickel, silver, nickel, zinc, tin,palladium, lead, copper, aluminum, molybdenum, titanium, iron,zirconium, magnesium, and alloys thereof). Particularly suitableconductive metals include, for instance, copper, copper alloys (e.g.,copper-zirconium, copper-magnesium, copper-zinc, or copper-iron),nickel, and nickel alloys (e.g., nickel-iron). The thickness of theterminations is generally selected to minimize the thickness of thecapacitor. For instance, the thickness of the terminations may rangefrom about 0.05 to about 1 millimeter, in some embodiments from about0.05 to about 0.5 millimeters, and from about 0.07 to about 0.2millimeters. One exemplary conductive material is a copper-iron alloymetal plate available from Wieland (Germany). If desired, the surface ofthe terminations may be electroplated with nickel, silver, gold, tin,etc. as is known in the art to ensure that the final part is mountableto the circuit board. In one particular embodiment, both surfaces of theterminations are plated with nickel and silver flashes, respectively,while the mounting surface is also plated with a tin solder layer.

The terminations may be connected to the capacitor element using anytechnique known in the art. In one embodiment, for example, a lead framemay be provided that defines the cathode termination and anodetermination. To attach the capacitor element to the lead frame, aconductive adhesive may initially be applied to a surface of the cathodetermination. The conductive adhesive may include, for instance,conductive metal particles contained with a resin composition. The metalparticles may be silver, copper, gold, platinum, nickel, zinc, bismuth,etc. The resin composition may include a thermoset resin (e.g., epoxyresin), curing agent (e.g., acid anhydride), and coupling agent (e.g.,silane coupling agents). Suitable conductive adhesives may be describedin U.S. Patent Application Publication No. 2006/0038304 to Osako, et al.Any of a variety of techniques may be used to apply the conductiveadhesive to the cathode termination. Printing techniques, for instance,may be employed due to their practical and cost-saving benefits. Theanode lead may also be electrically connected to the anode terminationusing any technique known in the art, such as mechanical welding, laserwelding, conductive adhesives, etc. Upon electrically connecting theanode lead to the anode termination, the conductive adhesive may then becured to ensure that the electrolytic capacitor is adequately adhered tothe cathode termination.

Referring to FIG. 1, for example, a capacitor assembly 30 is shown asincluding an anode termination 62 and a cathode termination 72 inelectrical connection with a capacitor element 33. Although it may be inelectrical contact with any of the surfaces of the capacitor element 33,the cathode termination 72 in the illustrated embodiment is inelectrical contact with the lower surface 39 via a conductive adhesive.More specifically, the cathode termination 72 contains a first component73 that is in electrical contact and generally parallel with the lowersurface 39 of the capacitor element 33. The cathode termination 72 mayalso contain a second component 74 that is substantially perpendicularto the first component 73 and in electrical contract with a rear surface38 of the capacitor element 33. The anode termination 62 likewisecontains a first component 63 positioned substantially perpendicular toa second component 64. The first component 63 is in electrical contactand generally parallel with the lower surface 39 of the capacitorelement 33. The second component 64 contains a region 51 that carries ananode lead 16. Although not depicted in FIG. 1, the region 51 maypossess a “U-shape” to further enhance surface contact and mechanicalstability of the lead 16.

The terminations may be connected to the capacitor element using anytechnique known in the art. In one embodiment, for example, a lead framemay be provided that defines the cathode termination 72 and anodetermination 62. To attach the capacitor element 33 to the lead frame,the conductive adhesive may initially be applied to a surface of thecathode termination 72. In one embodiment, the second component 64 ofthe anode termination 62 is initially bent upward to the position shownin FIG. 1. Thereafter, the capacitor element 33 is positioned on thecathode termination 72 so that its lower surface 39 contacts theadhesive and the anode lead 16 is received by the region 51. If desired,an insulating material (not shown), such as a plastic pad or tape, maybe positioned between the lower surface 39 of the capacitor element 33and the first component 63 of the anode termination 62 to electricallyisolate the anode and cathode terminations.

The anode lead 16 is then electrically connected to the region 51 usingany technique known in the art, such as mechanical welding, laserwelding, conductive adhesives, etc. For example, the anode lead 16 maybe welded to the anode termination 62 using a laser. Lasers generallycontain resonators that include a laser medium capable of releasingphotons by stimulated emission and an energy source that excites theelements of the laser medium. One type of suitable laser is one in whichthe laser medium consist of an aluminum and yttrium garnet (YAG), dopedwith neodymium (Nd). The excited particles are neodymium ions Nd³⁺. Theenergy source may provide continuous energy to the laser medium to emita continuous laser beam or energy discharges to emit a pulsed laserbeam. Upon electrically connecting the anode lead 16 to the anodetermination 62, the conductive adhesive may then be cured. For example,a heat press may be used to apply heat and pressure to ensure that theelectrolytic capacitor element 33 is adequately adhered to the cathodetermination 72 by the adhesive.

III. Casing Material

The capacitor element is generally encapsulated with a casing materialso that at least a portion of the anode and cathode terminations areexposed for mounting onto a circuit board. As shown in FIG. 1, forinstance, the capacitor element 33 may be encapsulated within a casingmaterial 28 so that a portion of the anode termination 62 and a portionof the cathode termination 72 are exposed.

As indicated above, the casing material contains an epoxy compositionthat comprises one or more inorganic oxide fillers and a resinousmaterial that includes one more epoxy resins that are crosslinked with aco-reactant (hardener). To help improve the overall moisture resistanceof the casing material, the content of the inorganic oxide fillers ismaintained at a high level, such as about 75 wt. % or more, in someembodiments about 76 wt. % or more, and in some embodiments, from about77 wt. % to about 90 wt. % of the composition. The nature of theinorganic oxide fillers may vary, such as silica, alumina, zirconia,magnesium oxides, iron oxides (e.g., iron hydroxide oxide yellow),titanium oxides (e.g., titanium dioxide), zinc oxides (e.g., boron zinchydroxide oxide), copper oxides, zeolites, silicates, clays (e.g.,smectite clay), etc., as well as composites (e.g., alumina-coated silicaparticles) and mixtures thereof. Regardless of the particular fillersemployed, however, a substantial portion, if not all, of the inorganicoxide fillers is typically in the form of vitreous silica, which isbelieved to further improve the moisture resistance of the casingmaterial due to its high purity and relatively simple chemical form.Vitreous silica may, for instance, constitute about 30 wt. % or more, insome embodiments from about 35 wt. % to about 90 wt. %, and in someembodiments, from about 40 wt. % to about 80 wt. % of the total weightof fillers employed in the composition, as well as from about 20 wt. %to about 70 wt. %, in some embodiments from about 25 wt. % to about 65wt. %, and in some embodiments, from about 30 wt. % to about 60 wt. % ofthe entire composition. Of course, other forms of silica may also beemployed in combination with the vitreous silica, such as quartz, fumedsilica, cristabolite, etc.

The resinous material typically constitutes from about 0.5 wt. % toabout 25 wt. %, in some embodiments from about 1 wt. % to about 24 wt.%, and in some embodiments, from about 10 wt. % to about 23 wt. % of thecomposition. Generally speaking, any of a variety of different types ofepoxy resins may be employed in the present invention. Examples ofsuitable epoxy resins include, for instance, bisphenol A type epoxyresins, bisphenol F type epoxy resins, phenol novolac type epoxy resins,orthocresol novolac type epoxy resins, brominated epoxy resins andbiphenyl type epoxy resins, cyclic aliphatic epoxy resins, glycidylester type epoxy resins, glycidylamine type epoxy resins, cresol novolactype epoxy resins, naphthalene type epoxy resins, phenol aralkyl typeepoxy resins, cyclopentadiene type epoxy resins, heterocyclic epoxyresins, etc. To help provide the desired degree of moisture resistance,however, it is particularly desirable to employ epoxy phenol novolac(“EPN”) resins, which are glycidyl ethers of phenolic novolac resins.These resins can be prepared, for example, by reaction of phenols withan excess of formaldehyde in the presence of an acidic catalyst toproduce the phenolic novolac resin. Novolac epoxy resins are thenprepared by reacting the phenolic novolac resin with epichlorihydrin inthe presence of sodium hydroxide. Specific examples of the novolac-typeepoxy resins include a phenol-novolac epoxy resin, cresol-novolac epoxyresin, naphthol-novolac epoxy resin, naphthol-phenol co-condensationnovolac epoxy resin, naphthol-cresol co-condensation novolac epoxyresin, brominated phenol-novolac epoxy resin, etc. Regardless of thetype of resin selected, the resulting phenolic novolac epoxy resinstypically have more than two oxirane groups and can be used to producecured coating compositions with a high crosslinking density, which canbe particularly suitable for enhancing moisture resistance. One suchphenolic novolac epoxy resin is poly[(phenyl glycidylether)-co-formaldehyde]. Other suitable resins are commerciallyavailable under the trade designation ARALDITE (e.g., GY289, EPN 1183,EP 1179, EPN 1139, and EPN 1138) from Huntsman.

As indicated, the epoxy resin is crosslinked with a co-reactant(hardener) to further improve the mechanical properties of thecomposition and also enhance its overall moisture resistance as notedabove. Examples of such co-reactants may include, for instance,polyamides, amidoamines (e.g., aromatic amidoamines such asaminobenzamides, aminobenzanilides, and am inobenzenesulfonam ides),aromatic diamines (e.g., diaminodiphenylmethane, diaminodiphenylsulfone,etc.), aminobenzoates (e.g., trimethylene glycol di-p-aminobenzoate andneopentyl glycol di-p-amino-benzoate), aliphatic amines (e.g.,triethylenetetramine, isophoronediamine), cycloaliphatic amines (e.g.,isophorone diamine), imidazole derivatives, guanidines (e.g.,tetramethylguanidine), carboxylic acid anhydrides (e.g.,methylhexahydrophthalic anhydride), carboxylic acid hydrazides (e.g.,adipic acid hydrazide), phenolic-novolac resins (e.g., phenol novolac,cresol novolac, etc.), carboxylic acid amides, etc., as well ascombinations thereof. Phenolic-novolac resins may be particularlysuitable for use in the present invention.

Apart from the components noted above, it should be understood thatstill other additives may also be employed in the epoxy composition usedto form the casing, such as photoinitiators, viscosity modifiers,suspension aiding agents, pigments, stress reducing agents, couplingagents (e.g., silane coupling agents), stabilizers, etc. When employed,such additives typically constitute from about 0.1 to about 20 wt. % ofthe total composition.

The particular manner in which the casing material is applied to thecapacitor body may vary as desired. In one particular embodiment, thecapacitor element is placed in a mold and the casing material is appliedto the capacitor element so that it occupies the spaces defined by themold and leaves exposed at least a portion of the anode and cathodeterminations. The casing material may be initially provided in the formof a single or multiple compositions. For instance, a first compositionmay contain the epoxy resin and the second composition may contain theco-reactant. Regardless, once it is applied, the casing material may beheated or allowed to stand at ambient temperatures so that the epoxyresin is allowed to crosslink with the co-reactant, which thereby causesthe epoxy composition to cure and harden into the desired shape of thecase. For instance, the composition may be heated to a temperature offrom about 15° C. to about 150° C., in some embodiments from about 20°C. to about 120° C., and in some embodiments, from about 25° C. to about100° C.

Although by no means required, a moisture barrier layer may also beemployed that coats all or a portion of the casing material. Themoisture barrier layer is generally formed from a hydrophobic elastomer,such as silicones, fluoropolymers, etc. Silicone elastomers areparticularly suitable for use in the moisture barrier layer of thepresent invention. Such elastomers are typically derived frompolyorganosiloxanes, such as those having the following general formula:

wherein,

x is an integer greater than 1; and

R₁, R₂, R₃, R₄, R₅, R₆, R₇, and R₈ are independently monovalent groupstypically containing from 1 to about 20 carbon atoms, such as alkylgroups (e.g., methyl, ethyl, propyl, pentyl, octyl, undecyl, octadecyl,etc.); alkoxy groups (e.g., methoxy, ethoxy, propoxy, etc.);carboxyalkyl groups (e.g., acetyl); cycloalkyl groups (e.g.,cyclohexyl); alkenyl groups (e.g., vinyl, allyl, butenyl, hexenyl,etc.); aryl groups (e.g., phenyl, tolyl, xylyl, benzyl, 2-phenylethyl,etc.); and halogenated hydrocarbon groups (e.g., 3,3,3-trifluoropropyl,3-chloropropyl, dichlorophenyl, etc.). Examples of suchpolyorganosiloxanes may include, for instance, polydimethylsiloxane(“PDMS”), polymethylhydrogensiloxane, dimethyidiphenylpolysiloxane,dimethyl/methylphenylpolysiloxane, polymethylphenylsiloxane,methylphenyl/dimethylsiloxane, vinyldimethyl terminatedpolydimethylsiloxane, vinylmethyl/dimethylpolysiloxane, vinyldimethylterminated vinylmethyl/dimethylpolysiloxane, divinylmethyl terminatedpolydimethylsiloxane, vinylphenylmethyl terminated polydimethylsiloxane,dimethylhydro terminated polydimethylsiloxane,methylhydro/dimethylpolysiloxane, methylhydro terminatedmethyloctylpolysiloxane, methylhydro/phenylmethyl polysiloxane,fluoro-modified polysiloxane, etc. To form an elastomer, thepolyorganosiloxane may be crosslinked using any of a variety of knowntechniques, such as by catalyst curing (e.g., platinum catalysts), roomtemperature vulcanization, moisture curing, etc. Crosslinking agents maybe employed, such as alkoxy silanes having the formula Si—OR, wherein Ris H, alkyl (e.g., methyl), alkenyl, carboxyalkyl (e.g., acetyl), and soforth.

In addition to being hydrophobic, it is generally desired that thematerial used to form the moisture barrier layer has a relatively lowmodulus and a certain degree of flexibility, which can help absorb someof the thermal stresses caused by expansion of the casing and also allowit to be subjected to compressive forces. The flexibility of thematerial may be characterized by a corresponding low modulus ofelasticity (“Young's modulus”), such as about 5,000 kilopascals (“kPa”)or less, in some embodiments from about 1 to about 2,000 kPa, and insome embodiments, from about 2 to about 500 kPa, measured at atemperature of about 25° C. The material also typically possesses acertain degree of strength that allows it to retain its shape even whensubjected to compressive forces. For example, the material may possess atensile strength of from about 1 to about 5,000 kPa, in some embodimentsfrom about 10 to about 2,000 kPa, and in some embodiments, from about 50to about 1,000 kPa, measured at a temperature of about 25° C. With theconditions noted above, the hydrophobic elastomer can even furtherenhance the ability of the capacitor to function under extremeconditions.

To help achieve the desired flexibility and strength properties, anon-conductive filler may be employed in the moisture barrier layer.When employed, such additives typically constitute from about 0.5 wt. %to about 30 wt. %, in some embodiments from about 1 wt. % to about 25wt. %, and in some embodiments, from about 2 wt. % to about 20 wt. % ofthe moisture barrier layer. The silicone elastomer may constitute fromabout 70 wt. % to about 99.5 wt. %, in some embodiments from about 75wt. % to about 99 wt. %, and in some embodiments, from about 80 wt. % toabout 98 wt. % of the moisture barrier layer. One particular example ofsuch a filler includes, for instance, silica. While most forms of silicacontain a relatively hydrophilic surface due to the presence of silanolgroups (Si—OH), the silica may optionally be surface treated so that itssurface contains (CH₃)_(n)—Si— groups, wherein n is an integer of 1 to3, which further enhances the hydrophobicity of the moisture barrierlayer. The surface treatment agent may, for example, be an organosiliconcompound monomer having a hydrolyzable group or a partial hydrolyzatethereof. Examples of such compounds may include organosilazanes, silanecoupling agents such as described above, etc.

Due to its unique construction, the resulting capacitor assembly canexhibit a variety of beneficial properties. For example, the dissipationfactor of the capacitor assembly may be maintained at relatively lowlevels. The dissipation factor generally refers to losses that occur inthe capacitor and is usually expressed as a percentage of the idealcapacitor performance. For example, the dissipation factor of thecapacitor of the present invention is typically from about 1% to about25%, in some embodiments from about 3% to about 10%, and in someembodiments, from about 5% to about 15%, as determined at a frequency of120 Hz. The capacitor assembly may also be able to be employed in highvoltage applications, such as at rated voltages of about 35 volts ormore, in some embodiments about 50 volts or more, and in someembodiments, from about 60 volts to about 200 volts. The capacitorassembly may, for example, exhibit a relatively high “breakdown voltage”(voltage at which the capacitor fails), such as about 2 volts or more,in some embodiments about 5 volts or more, in some embodiments about 10volts or more, and in some embodiments, from about 10 to about 100volts. Likewise, the capacitor assembly may also be able to withstandrelatively high surge currents, which is also common in high voltageapplications. The peak surge current may be, for example, about 100 Ampsor more, in some embodiments about 200 Amps or more, and in someembodiments, and in some embodiments, from about 300 Amps to about 800Amps.

The present invention may be better understood by reference to thefollowing examples.

Test Procedures

Capacitance

The capacitance may be measured using a Keithley 3330 Precision LCZmeter with Kelvin Leads with 2.2 volt DC bias and a 0.5 volt peak topeak sinusoidal signal. The operating frequency may be 120 Hz and thetemperature may be 23° C.±2° C. In some cases, the “wet-to-dry”capacitance can be determined. The “dry capacitance” refers to thecapacitance of the part before application of the solid electrolyte,graphite, and silver layers, while the “wet capacitance” refers to thecapacitance of the part after formation of the dielectric, measured in14% nitric acid in reference to 1 mF tantalum cathode with 10 volt DCbias and a 0.5 volt peak to peak sinusoidal signal after 30 seconds ofelectrolyte soaking.

Equivalent Series Resistance (ESR)

Equivalence series resistance may be measured using a Keithley 3330Precision LCZ meter with Kelvin Leads 2.2 volt DC bias and a 0.5 voltpeak to peak sinusoidal signal. The operating frequency may 100 kHz andthe temperature may be 23° C.±2° C.

Humidity Testing

Humidity testing may be conducted (25 parts) at a temperature of 85° C.,85% relative humidity, and at the rated voltage (e.g., 16 volts).Capacitance and ESR can be recorded after 120, 500, and 1,000 hours atrecovered samples and then compared to the initial measurement at 0hours. The recovery time after the test conditions may be from 6 to 24hours.

EXAMPLE 1

150,000 μFV/g tantalum powder was used to form anode samples. Each anodesample was embedded with a tantalum wire, sintered at 1300° C., andpressed to a density of 6.5 g/cm³. The resulting pellets had a size of2.10×1.25×0.70 mm. The pellets were anodized to 13.0 volts inwater/phosphoric acid electrolyte with a conductivity of 8.6 mS at atemperature of 85° C. to form the dielectric layer. The pellets wereanodized again to 70 volts in a water/boric acid/disodium tetraboratewith a conductivity of 2.0 mS at a temperature of 30° C. for 25 secondsto form a thicker oxide layer built up on the outside.

A conductive polymer coating was then formed by dipping the anode into abutanol solution of iron (III) toluenesulfonate (Clevios™ C, H.C.Starck) for 5 minutes and consequently into 3,4-ethylenedioxythiophene(Clevios™ M, H.C. Starck) for 1 minute. After 45 minutes ofpolymerization, a thin layer of poly(3,4-ethylenedioxythiophene) wasformed on the surface of the dielectric. The anode was washed inmethanol to remove reaction by-products, anodized in a liquidelectrolyte, and washed again in methanol. This process was repeated 6times. Thereafter, the parts were dipped into a dispersedpoly(3,4-ethylenedioxythiophene) having a solids content 2.0% andviscosity 20 mPa·s (Clevios™ K, Heraeus). Upon coating, the parts weredried at 125° C. for 20 minutes. This process was repeated 3 times.Thereafter, the parts were dipped into a dispersedpoly(3,4-ethylenedioxythiophene) having a solids content of 2% andviscosity 160 mPa·s (Clevios™ K, Heraeus). Upon coating, the parts weredried at 125° C. for 20 minutes. This process was repeated 8 times. Theparts were then dipped into a graphite dispersion and dried. Finally,the parts were dipped into a silver dispersion and dried. Multiple parts(4000) of 150 μF/6.3V capacitors were made in this manner andencapsulated in a silica resin.

EXAMPLE 2

Capacitors were formed in the manner described in Example 1, except thata hydrophobic casing material is employed as described herein. Multipleparts (4000) of 150 μF/6.3V capacitors were formed.

25 parts of finished capacitors of Examples 1-2 were then tested forelectrical performance. The median results (first quartile (“Q1”),median and third quartile (“Q3”)) of capacitance (CAP) and ESR within85° humidity testing at rated voltage are set forth below in Table 1 andTable 2.

TABLE 1 Humidity Testing Results (CAP) Time. Q1 - CAP median - CAP Q3 -CAP [h] [μF] [μF] [μF] Example 1 0 140.12 140.69 141.87 500 149.74153.59 154.49 Example 2 0 138.90 139.65 140.29 500 150.99 153.41 154.43

TABLE 2 Humidity Testing Results (ESR) Time. Q1 - ESR median - ESR Q3 -ESR [h] [Ohms] [Ohms] [Ohms] Example 1 0 0.0414 0.0464 0.0533 500 0.04260.0465 0.0923 Example 2 0 0.0408 0.0431 0.0448 500 0.0419 0.0447 0.0470

These and other modifications and variations of the present inventionmay be practiced by those of ordinary skill in the art, withoutdeparting from the spirit and scope of the present invention. Inaddition, it should be understood that aspects of the variousembodiments may be interchanged both in whole or in part. Furthermore,those of ordinary skill in the art will appreciate that the foregoingdescription is by way of example only, and is not intended to limit theinvention so further described in such appended claims.

What is claimed is:
 1. A capacitor assembly comprising: a solidelectrolytic capacitor element that contains a sintered porous anodebody wherein the anode body comprises tantalum, a dielectric thatoverlies the anode body, and a solid electrolyte that overlies thedielectric; an anode termination that is in electrical connection withthe anode body; a cathode termination that is in electrical connectionwith the solid electrolyte; and a casing material that encapsulates thecapacitor element and leaves exposed at least a portion of the anodetermination and the cathode termination, wherein the casing material isformed from an epoxy composition that contains one or more inorganicoxide fillers and a resinous material that includes one or more epoxyresins crosslinked with a co-reactant, wherein inorganic oxide fillersconstitute about 75 wt. % or more of the epoxy composition, and vitreoussilica constitutes about 30 wt. % or more of the total weight ofinorganic oxide fillers in the epoxy composition.
 2. The capacitorassembly claim 1, wherein vitreous silica constitutes from about 20 wt.% to about 70 wt. % of the epoxy composition.
 3. The capacitor assemblyof claim 1, wherein the resinous material constitutes from about 0.5 wt.% to about 25 wt. % of the epoxy composition.
 4. The capacitor assemblyof claim 1, wherein the epoxy resin is a phenolic novolac epoxy resin.5. The capacitor assembly of claim 1, wherein the phenolic novolac epoxyresin is a phenol-novolac epoxy resin, cresol-novolac epoxy resin,naphthol-novolac epoxy resin, naphthol-phenol co-condensation novolacepoxy resin, naphthol-cresol co-condensation novolac epoxy resin,brominated phenol-novolac epoxy resin, or a combination thereof.
 6. Thecapacitor assembly of claim 1, wherein the co-reactant is a polyamide,amidoamines, aromatic diamine, aminobenzoates, aliphatic amines,cycloaliphatic amines, imidazole derivative, guanidine, carboxylic acidanhydrides, carboxylic acid hydrazides, phenolic-novolac resin,carboxylic acid amide, or a combination thereof.
 7. The capacitorassembly of claim 1, wherein the co-reactant is a phenolic-novolacresin.
 8. The capacitor assembly of claim 1, wherein the capacitorelement further comprises a cathode coating that contains a metalparticle layer that overlies the solid electrolyte, wherein the metalparticle layer includes a plurality of conductive metal particlesdispersed within a resinous polymer matrix.
 9. The capacitor assembly ofclaim 8, wherein the metal particles include silver.
 10. The capacitorassembly of claim 1, wherein the solid electrolyte includes a pluralityof conductive polymer particles.
 11. The capacitor assembly of claim 10,wherein the conductive polymer particles contain an extrinsicallyconductive polymer having repeating units of the following formula(III):

wherein, R₇ is a linear or branched, C₁ to C₁₈ alkyl radical, C₅ to C₁₂cycloalkyl radical, C₆ to C₁₄ aryl radical, C₇ to C₁₈ aralkyl radical,or a combination thereof; and q is an integer from 0 to
 8. 12. Thecapacitor assembly of claim 11, wherein the extrinsically conductivepolymer is poly(3,4-ethylenedioxythiophene).
 13. The capacitor assemblyof claim 11, wherein the particles also contain a polymeric counterion.14. The capacitor assembly of claim 10, wherein the conductive polymerparticles contain an intrinsically conductive polymer having repeatingunits of the following formula (IV):

wherein, R is (CH₂)_(a)—O—(CH₂)_(b); a is from 0 to 10; b is from 1 to18; Z is an anion; X is a cation.
 15. The capacitor assembly of claim 1,further comprising an external polymer coating that overlies the solidelectrolyte and contains pre-polymerized conductive polymer particlesand a cross-linking agent.
 16. The capacitor assembly of claim 1,wherein the capacitor is in contact with an atmosphere having a relativehumidity of about 40% or more.
 17. The capacitor assembly of claim 1,further comprising a moisture barrier layer that coats at least aportion of the casing material.
 18. The capacitor assembly of claim 17,wherein the moisture barrier layer includes a silicone elastomer.